Low aerobic fitness during late adolescence increases the risk for early death, according to a study published online December 20 in the International Journal of Epidemiology.

The results also undermine the "fat but fit" concept by showing that physically fit obese men are at higher risk for death than unfit normal-weight men.

"In the present study, we found a graded association between aerobic fitness at the age of 18 years and the risk of early death," write Gabriel Hogstrom, PhD, a postgraduate student in the Department of Community Medicine and Rehabilitation at Umea University, Sweden, and colleagues.

"Also of interest was the finding that the risk of death from any cause during follow-up was 30% lower in unfit normal-weight men than in obese fit men," they continued. "These results counter the notion that the 'fat but fit' condition does not increase mortality risk."

Much research supports the idea that frequent physical activity decreases the risk for death. Research on the detrimental health effects of low fitness levels, however, has focused mainly on older populations. Few studies have looked at the direct link between aerobic fitness and health in younger individuals.

The study drew data from the Swedish Military Conscription Registry and included 1,317,713 Swedish men (mean age, 18 years) conscripted into the Swedish military between 1969 and 1996. At the time of conscription, the men underwent baseline assessments that included aerobic fitness testing, in which they cycled until fatigue caused them to stop. The researchers looked at all-cause and cause-specific death, using national registers. Mortality information came from the National Cause of Death Registry.

During a mean follow-up of 28.8 years, 44,301 of the men died.

After adjusting for age and conscription year, men with the highest aerobic fitness levels had 51% lower risk for all-cause death (hazard ratio [HR], 0.49; 95% con?dence interval [CI], 0.47 - 0.51) compared with those with the lowest fitness levels. Similar findings resulted from analyses of weight-adjusted fitness (HR, 0.52; 95% CI, 0.50 - 0.54).

Obese men benefited less than normal-weight men from being physically fit (P interaction < .001). Normal-weight men in all levels of aerobic fitness had lower risk for all-cause mortality (30% - 48%; P < .05 for all) compared with obese men with the highest levels of aerobic fitness.

The benefits of aerobic fitness decreased as weight increased. After adjusting for age and year, normal-weight men in the upper half of aerobic fitness had 34% lower risk for death than those in the lower half (HR, 0.66; 95% CI, 0.64 - 0.68). This benefit decreased to 28% in overweight men, and disappeared entirely in men with the highest levels of obesity.

This finding "challenges the currently held idea that obese individuals can fully compensate mortality risk by being physically fit," the authors write.

"Despite the limitation posed by the observational nature of this study, these results suggest that low [body mass index] early in life is more important than high physical fitness, with regard to reducing the risk [for] early death," they conclude.

"Finally, associations were observed between fitness and the risk of early death in lower [body mass index] categories, but not in the obese category, adding to the evidence against the existence of healthy obesity."